Nonwoven fabrics having flame retardancy are employed extremely effectively in the fields of general industrial materials, electric and electronic materials, medical materials, agricultural materials, optical materials, materials for aircrafts, automobiles, and ships, apparel and the like, in particular in applications in which there are many opportunities of exposure to high-temperature environments.
In recent years, a nonwoven fabric using split fibers and a nonwoven fabric made of extra-fine fibers manufactured with a flash spinning method, a melt blown method, or the like have been developed, and are used for filter application and the like. However, such a nonwoven fabric made of extra-fine fibers is mainly composed of a resin such as polypropylene or polyethylene terephthalate, and hence flame retardancy and heat resistance have been insufficient and use thereof at a high temperature has not been suitable.
Although some techniques for manufacturing a nonwoven fabric using fibers made of a flame retardant polymer have been attempted, such an unfavorable condition as melt fracture or high melt tension takes place in an attempt to obtain extra-fine fibers, and it has been difficult to obtain a nonwoven fabric made of flame retardant extra-fine fibers with good productivity.
The applicant has proposed, as a nonwoven fabric made of polyetherimide (hereinafter also referred to as “PEI”) fibers having flame retardancy, a nonwoven fabric mainly composed of amorphous PEI fibers having a specific structure and three-dimensionally entangled with one another, for example in Japanese Patent Laying-Open No. 2012-41644 (PTD 1). Regarding amorphous PEI fibers, the applicant has also proposed heat-resistant and flame-retardant paper having not only excellent flame retardancy and heat resistance but also low equilibrium moisture regain in Japanese Patent Laying-Open No. 2011-127252 (PTD 2), and a heat fusion fiber, a fiber structure body, and a heat-resistant molded body having excellent heat resistance, flame retardancy, and dimensional stability in International Publication No. 2012/014713 (PTD 3).
Thus, amorphous PEI fibers are not only high in melting point and excellent in heat resistance owing to its molecular frame but also excellent in flame retardancy. However, examples in PTD 1 disclose only a nonwoven fabric made with a spun lace method, which has a relatively high fineness with a fiber diameter being 2.2 dtex (corresponding to 15 μm). Although a nonwoven fabric made of amorphous PEI fibers and having denseness increased enough to have electrical insulation has not been known, if it is possible to provide a nonwoven fabric which has electrical insulation in addition to flame retardancy, such a nonwoven fabric is expected to be applicable to wider applications, such as the field of electrical insulating paper.